The present invention relates to a novel process for forming a retroreflective surface on a substrate using a modified dry powder coating process. More particularly, the present invention relates to a process for applying optical elements and a binder material to a substrate to form a retroreflective surface on road markers, airport runways, or signs and the like using a dry powder coating process.
Most highway guidance lines, such as centerlines, edge lines and lane markers, depend upon some sort of light-reflecting device for making them more visible at night when the only source of illumination is the light from the motor vehicle head lamps. Such reflecting devices can be cube corners, glass microspheres, or simply light colored objects protruding above the pavement surface.
A plain white line painted on the surface or even a plain white plastic line adhered to the road surface is not easily visible even at a distance as near as 100 feet because of the extremely shallow angle of the light emanating from vehicle head lamps which impinge upon the road surface. Most of the incidental light is scattered and thus reflected away from the vehicle and very little returns by reflection for the operator to detect. Use of light-reflecting devices such as those mentioned above, incorporated within the painted or other light-colored line, can increase a motorists detection of the line out to many hundreds of feet. For example, the incorporation of transparent glass microspheres, ranging in size from a few thousandths of an inch in diameter to as much as a tenth of an inch, produce a better light reflection through an effect in which the microspheres serve as miniature optical lenses which focus the incident light from the headlamps into a tiny spot located a slight distance behind the rear surface of the microspheres. The focused spot of light falling upon a pigmented material after undergoing scattering is then partially reflected back upon itself and reaches the motorist""s eyes by a phenomenon called retro-reflection. Because of light scattering by the pigmented binder in which the microspheres are partially embedded, only a small percentage of the incident light is returned by retro-reflection; but even this is considerably more light than is the case of an ordinary painted line. During daylight, the ordinary painted line is easily seen by a motorist for thousands of feet because of the abundance of ambient overhead skylight incident upon the line.
The principle of using glass microspheres as light-reflecting lenses for highway markers was disclosed as early as 1936 in U.S. Pat. No. 2,043,414. Soda-lime-silicate glass, such as window glass with a refractive index of 1.5, is commonly used as the medium for the microspheres because it is relatively inactive chemically and is a very hard material. This glass, forming the microspheres, generally causes the incident light to come to a focus some distance behind the rear surface of the microsphere. An increase in the brightness can result, however, when the light comes to a focus upon the rear surface of the microsphere itself. This occurs when a glass with a higher index of refraction is used. The distance behind the rear surface of a glass microsphere where the incident light comes to a focus is a function of the refractive index of the glass. As the refractive index increases from a value of approximately 1.5, the focus point moves in closer to the rear surface of the microsphere, reaching this surface when a refractive index value of approximately 1.9 is attained. At this point, the majority of the incident light is returned back upon itself in a retro-reflected beam.
If the rear surface of a microsphere is covered with a highly specular light-reflecting metal such as aluminum, chromium, silver or some other specularly reflective material, then the entire incident light beam is returned except for small losses due to absorption and other minor effects, such as spherical aberration. Even without such a reflective coating, however, the returned light beam is considerably brighter than it would be with a lower refractive index glass. This effect is achieved because the scattered light in the focused spot is very near the rear surface of the sphere itself and thus most of it re-enters the sphere and produces a brilliant retroreflected beam.
All known techniques for producing a retroreflective surface on a substrate using reflective elements embedded in a binder utilize conventional coating techniques such as painting, laminating or dipping of the substrate in the binder. Such techniques are relatively expensive, inefficient and generate a large amount of waste and pollution.
Electrostatic powder coating is a technique whereby an electrostatically charged particulate is adhered to an exposed surface of a neutrally charged object. This particulate can comprise any of a number of compounds, including a variety of thermoset and thermoplastic materials. The charged particles adhere to the surface of the object and are subsequently permanently bonded thereto by curing the powder coating using heat or some other method. The resulting coating provides exceptional toughness and impact resistance as well as resistance to environmental and chemical exposure. Fluidized bed powder coating is a technique in which powder particles are dispersed throughout a chamber by low pressure air or other gas. When a preheated substrate is introduced into the chamber, the particles strike the substrate where they melt and cling to its surface. Subsequent curing of the melted particles permanently bonds them to the substrate.
The use of powdered coating techniques for coating and coloring the exposed surfaces of finished articles has increased in recent years, taking the place of traditional painting and dipping techniques. Powder coating techniques offer numerous advantages over conventional coating processes utilizing paint, lacquer or other solvent-based carriers.
A first, and perhaps the most important advantage, is the fact that powder coatings are applied without the use of solvents, thereby greatly reducing the amount of polluting volatile organic compounds released into the atmosphere. This allows the coating industry to meet ever increasingly strict environmental regulations and worker safety concerns easily and inexpensively. This aspect of powder coating, along with the fact that excess powder spray can be collected for reuse, also reduces the cost of disposal of potentially hazardous and flammable waste.
Thus, because powder coating provides many advantages over traditional coating techniques, a need exists for a method of producing retroreflective surfaces on a substrate utilizing reflective elements in a powder coating process.
The process of the present invention involves coating a substrate or material with both a powder coating material and a retroreflective material. The powder coating material provides a tough, corrosion resistant protective layer on the substrate and also acts as a binder in which the retroreflective material is subsequently embedded. The powder coating process of the present invention includes the following steps:
1. Pre-treatment of the substrate
2. Drying of the substrate
3. Powder coating
4. Semi-cure the powder coating
5. Apply retroreflective layer
6. Finish cure powder coating
7. Apply clear coat or translucent composition (optional step)
Pretreatment of the substrate is typically necessary to ensure that the powder coating will adhere to the substrate. This pretreatment, which generally includes the steps of cleaning and conversion coating, helps to remove dirt, grease and oil from the substrate, as well as providing improved adhesion and corrosion resistance. After the substrate is dried, a powder coating is applied to its surface by one of several methods, such as electrostatic spray or fluidized bed treatment. Any of the various known powder coatings can be used in the powder coating process according to the present invention. Typical powders useable in the powder coating process include, but are not limited to, epoxy compounds, polyesters, acrylics, polyester urethanes, acrylic epoxies and various hybrids and combinations thereof. Partial curing of the powder coating is accomplished by conventional methods such as oven curing or curing with infrared radiation. This partially cured powder acts as a binder in which reflective elements can be subsequently embedded. The partial curing step comprises approximately 75% of the total cure time or to the gel point of the powder.
After the powder coating is partially cured, a retroreflective layer is applied to the substrate on top of the partially cured powder. The retroreflective layer is comprised of a reflective material. Suitable reflective materials for the retroreflective layer include glasses, ceramics, metal flakes, plastics and other reflective materials known in the art. The reflective material can be in the form of small beads or chips, collectively known as reflective elements, and can be applied in any conventional manner, such as fluidized bed or sprayed on methods. Suitable reflective elements in the present invention include ceramic or glass beads or microspheres. Typical of these include beads made from soda-lime-silicate glasses (also known as barium titanate glass beads).
After the reflective material is applied to the powder-coated substrate, the powder is fully cured to intimately bond the reflective material to the cured powder layer. An optional clear coat or translucent composition may subsequently be added to provide additional protection and adhesion.